It is known in the art to provide uninterruptable power supply (UPS) systems, which comprise at least one converter or multiple parallel converters. The converters are responsible for converting source power into output power as required for a load. Hence, the converters are typically connected to a primary power source for normal operation and a secondary power source, which powers the load in case of a failure of the primary power source. Primary and secondary source can be any kind of source, which can be provided individually for each converter, or which can be provided commonly for groups of converters or even all converters. Typically, the primary power source is an AC source, and the secondary power source is a DC source.
The typical design of a converter comprises a DC link, which is connected to a first and second input converter unit connected to the primary and secondary power source, respectively, and an output converter unit, which provides the output power as required for the load.
The design of the UPS system comprises AC/DC and DC/AC converters for historical reasons due to the simplicity and prevalence of AC power transmission. Nevertheless, the output converter unit can be a DC/AC-converter or a DC/DC-converter, depending on the type of load. An AC/DC-DC/AC conversion scheme with an AC load is industry standard but suffers from a penalty of not optimal system efficiency as typical loads comprise AC/DC-DC/DC converters. Accordingly, the DC/AC conversion at the load side of the converter prior to the AC/DC conversion at an input side of the load is not necessary and merely adds losses to the overall system efficiency.
For this reason, a new operating environment based on AC/DC-DC/DC UPS is emerging, thereby eliminating the above conversion steps from DC to AC at the converter and from AC to DC at the load, which are unnecessary conversion steps, so that the losses in the overall system are reduced. There is no standard yet established. However, practical alternatives are a two wire DC output of the UPS system at about 380V with either end connected to system reference, either a AC neutral or ground, or a split DC of the same voltage with midpoint connected to a reference. A drawback is loss of fault clear capability as a bypass with very high current source capability with a DC source is unpractical.
Fault clear capability refers to clearance of operating conditions of the load. The load side of the converter is current limited due to availability of practical semiconductor devices allowing a maximum current. Hence, abnormal operating conditions of the load, e.g. a short circuit, can make the UPS system to operate at maximum power without activation of protection means at the load side, e.g. without melting fuses which protect the load. Protection means of the UPS system can be activated prior to activating protection means at the load side, thereby leaving the load in a potentially dangerous state for the load and for operating personal.
In state of the art UPS systems, which provide AC power to the load, abnormal operating conditions, e.g. a short circuit, rely on a separate power path bypassing the converter units to connect the AC source directly to the load side to clear the fault through a current, which is limited only by source impedance.